Many detection systems for determining the presence or absence of a particular target analyte in a sample are known. Examples of detection systems for detecting analytes include immunoassays, such as an enzyme linked immunosorbent assays (ELISAs), which are used in numerous diagnostic, research and screening applications. Generally, these detection systems detect the target analyte when it binds to a specific binding agent or probe resulting in a measurable signal.
When using known detection systems, such as immunoassays, the ability to detect a target analyte is often limited by the low concentration of the target analyte in the sample and by non-specific interactions, such as non-specific binding of signal producing molecules and non-specific binding of sample molecules. The ability to detect a target analyte in a biological sample is often limited by these two factors.
The signal generated by detection systems is normally proportional to the number of target analytes that bind to the specific binding probe. Therefore, when the concentration of target is low, the signal is low. The total signal can be increased by increasing the signal associated with each bound target analyte. Often, detection systems use a solid support and reporter markers, such as fluorescent molecules, to generate the signal. Several strategies that use reporter markers have been designed to increase the signal associated with each bound target, such as in branched-DNA (Hendricks et al., Am J Clin Pathol. 1995, 104(5):537) and hybrid capture (WO 2003078966 A2). While these strategies increase the total signal, they often also increase the background noise resulting from the non-specific interaction between the reporter marker and the solid support. These strategies do not offer an effective method of discriminating reporter markers non-specifically bound to the solid support.
The use of micrometer scale particles as reporter markers, described in PCT/GB2010/001913, offers a method to remove particles non-specifically bound to the solid support by applying a controlled fluid drag force on the particles. However, the drag force significantly reduces the signal as well as the background noise because the disrupting force experienced by the target containing tethers is as high as the force experienced by the non-specific tethers.
Another strategy, disclosed in PCT/GB2010/001913 (WO 2011/045570 A2), uses a magnetic bead tethered to a solid support by an elongated molecule as a sensing apparatus to detect, for example a signal from an ELISA assay. According to this disclosure, the bead is tethered to the solid support independently of the presence or absence of target molecules and the signal is amplified be releasing manipulating agents that act on the elongated molecule. This strategy does not provide a simple method to discriminate non-specific interactions.
Accordingly, there is a need for a detection unit and systems of such units as well as methods capable of detecting low concentrations of target analytes while distinguishing non-specific binding from specific binding in the sample.